1. Field of the Invention
The present invention concerns a helmet and it particularly relates to a helmet which is worn as a safety cap for sport upon driving of a two wheeled vehicle or four wheeled vehicle and to such a helmet in which a shock absorbing liner is improved.
2. Statement of the Related Art
Heretofore, a helmet has a shell on an outer side and a shock absorbing liner disposed along the inside of the shell. As the shock absorbing liner, polystyrene (PS) foam, polypropylene (PP) foam and the like have been used but PS foam has drawbacks of undergoing large impact acceleration at high temperature, having poor restorability after releasing compression upon impact shock, and shrinking and being transformed at a high temperature of 70.degree. C. or higher. Further, PP foam has a drawback of undergoing a large reduction in compression strength at high temperature.
Further, in helmets using thermoplastic resins for the shell members, it has been difficult in view of strength or the like to satisfy JIS class C standards (Helmets for racing) due to the foregoing drawbacks of the shock absorbing liner. For satisfying the JIS C standards, FRP of high strength is used as the shell material or the thickness of the liner is increased for coping with the foregoing problems.
The related art, use of FRP of high strength as the shell material or increase in the thickness of the liner, results in drawbacks in that the shape of the helmet is enlarged, the helmet is made heavy, production cost is increased or users' demand can not be satisfied in view of the design.
By using polyvinylidene chloride (PVDC) foam as an impact shock absorbing liner, the above-mentioned drawbacks are able to be improved. However, since foaming gases of PVDC foam scarcely escape, an impact shock absorbing liner that is made of PVDC foam has a drawback of swelling at a high temperature of 70.degree. C. or higher. Taking the possible situation into consideration, that a helmet is left under the open sky or put in a helmet box of a motorcycle parked outdoors in midsummer, it is desirable that this drawback of PVDC foam is improved.
Further, since it takes a long period of time for foaming of PVDC foam, the cost of helmet production becomes expensive. Furthermore, since PVDC foam is manufactured with freon as a foaming agent and contains chlorine in its molecule, a substitution for PVDC foam is desired to be developed in view of an influence to the environment.